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T O P I C R E V I E W

spaceuk

Following eight years of capturing dramatic images and surprising science from Jupiter and its moons, NASA's Galileo mission draws to a close September 21 with a plunge into Jupiter's atmosphere.

The spacecraft will transmit a few hours of science measurements in real time, leading up to impact on Sunday, September 21.

From 4:00 to 5:00 p.m. EDT, September 21, JPL will provide live commentary from the mission control room and footage of the countdown clock as Galileo nears its final moments. The televised special will feature two panels. One will include former project managers, and the other former project scientists

I am not as up to speed on unmanned spaceflight as I am on manned. I have a question. Why is Galileo being impacted on Jupiter instead of Europa, and its possible subsurface ocean? Surely the hardy little craft could not damage Europa? Could it?

Ed

------------------"The heavens declare the glory of God; the skies proclaim the work of his hands." Psalms 19:1 NIV

micropooz

I've heard that the reason for the Jupiter entry was to ensure that Galileo does NOT impact one of the moons some day. The concern is that any hardy little earth organisms that may have survived on Galileo could contaminate a moon that may (admittedly this is a longshot) be capable of harboring life. So burning up in Jupiter's atmosphere should hopefully nuke any residual bugs and get a little more data on Jupiter's atmosphere before Galileo burns up.

Scott

Found this great URL that has "live" simulations of where Galileo is right now. The pictures below are linked directly from the website, they should be current to within 5 minutes. The first is an overhead view as Galileo crashes into Jupiter and the second is the way Jupiter looks from Galileo.

The Galileo spacecraft's 14-year odyssey came to an end on Sunday, Sept. 21, when the spacecraft passed into Jupiter's shadow then disintegrated in the planet's dense atmosphere at 11:57 a.m. Pacific Daylight Time. The Deep Space Network tracking station in Goldstone, Calif., received the last signal at 12:43:14 PDT. The delay is due to the time it takes for the signal to travel to Earth.

Hundreds of former Galileo project members and their families were present at NASA's Jet Propulsion Laboratory in Pasadena, Calif., for a celebration to bid the spacecraft goodbye.

Having traveled approximately 4.6 billion kilometers (about 2.8 billion miles), the hardy spacecraft endured more than four times the cumulative dose of harmful jovian radiation it was designed to withstand. During a previous flyby of the moon Amalthea in November 2002, flashes of light were seen by the star scanner that indicated the presence of rocky debris circling Jupiter in the vicinity of the small moon. Another measurement of this area was taken today during Galileo's final pass. Further analysis may help confirm or constrain the existence of a ring at Amalthea's orbit.

"We haven't lost a spacecraft, we've gained a steppingstone into the future of space exploration," said Dr. Torrance Johnson, Galileo project scientist.

The spacecraft was purposely put on a collision course with Jupiter because the onboard propellant was nearly depleted and to eliminate any chance of an unwanted impact between the spacecraft and Jupiter's moon Europa, which Galileo discovered is likely to have a subsurface ocean. Without propellant, the spacecraft would not be able to point its antenna toward Earth or adjust its trajectory, so controlling the spacecraft would no longer be possible. The possibility of life existing on Europa is so compelling and has raised so many unanswered questions that it is prompting plans for future spacecraft to return to the icy moon.

Galileo was launched from the cargo bay of Space Shuttle Atlantis in 1989. The exciting list of discoveries started even before Galileo got a glimpse of Jupiter. As it crossed the asteroid belt in October 1991, Galileo snapped images of Gaspra, returning the first ever close-up image of an asteroid. Less then a year later, the spacecraft got up close to yet another asteroid, Ida, revealing it had its own little "moon," Dactyl, the first known moon of an asteroid. In 1994 the spacecraft made the only direct observation of a comet impacting a planet-- comet Shoemaker-Levy 9's collision with Jupiter.

The descent probe made the first in-place studies of the planet's clouds and winds, and it furthered scientists' understanding of how Jupiter evolved. The probe also made composition measurements designed to assess the degree of evolution of Jupiter compared to the Sun.

Galileo made the first observation of ammonia clouds in another planet's atmosphere. It also observed numerous large thunderstorms on Jupiter many times larger than those on Earth, with lightning strikes up to 1,000 times more powerful than on Earth. It was the first spacecraft to dwell in a giant planet's magnetosphere long enough to identify its global structure and to investigate the dynamics of Jupiter's magnetic field. Galileo determined that Jupiter's ring system is formed by dust kicked up as interplanetary meteoroids smash into the planet's four small inner moons. Galileo data showed that Jupiter's outermost ring is actually two rings, one embedded within the other.

Galileo extensively investigated the geologic diversity of Jupiter's four largest moons: Ganymede, Callisto, Io and Europa. Galileo found that Io's extensive volcanic activity is 100 times greater than that found on Earth. The moon Europa, Galileo unveiled, could be hiding a salty ocean up to 100 kilometers (62 miles) deep underneath its frozen surface containing about twice as much water as all the Earth's oceans. Data also showed Ganymede and Callisto may have a liquid-saltwater layer. The biggest discovery surrounding Ganymede was the presence of a magnetic field. No other moon of any planet is known to have one.

The prime mission ended six years ago, after two years of orbiting Jupiter. NASA extended the mission three times to continue taking advantage of Galileo's unique capabilities for accomplishing valuable science. The mission was possible because it drew its power from two long-lasting radioisotope thermoelectric generators provided by the Department of Energy.

"The mission was a testimonial to the persistence of NASA even through tremendous challenges. It was a phenomenal mission," said Sean O'Keefe, NASA administrator.

BLACKARROW

Well done, all who were involved in Project Galileo, and well done Galileo spacecraft, a "...good and faithful servant."

To give some perspective to the duration of Project Galileo, from spacecraft construction to mission end, I was planning to watch the probe being launched on the shuttle. Two deep-space probes (Galileo and Ulysses)were going to be launched on two separate space shuttle flights a few days apart, in May, 1986, using Pads 39A and B. Sadly, as we know, the launches never took place and Galileo had to wait another three years to begin its journey.

Rizz

I think the reason for crashing Galileo into the planet Jupiter on purpose is this: Galileo since its launch has been powered by Plutonium-238 in a RTG or Radioisotope Thermal Generator.

48 pounds of Plutonium are (were) currently on board.

Plutonium-238 decays via alpha and gamma emission, its by products are also highly radioactive for a long time. An RTG catches this radiation in a heat exchanging mechanism and there converts it into electricity.

I think this was more of a concern than Earthly organisms, but I could be wrong.

Either way, it has been a wonderful journey.

Aloha Galileo!

Rizz

Robert Pearlman

quote:Originally posted by Rizz:I think the reason for crashing Galileo into the planet Jupiter on purpose is this: Galileo since its launch has been powered by Plutonium-238 in a RTG or Radioisotope Thermal Generator.

If we weren't concerned with contaminating Europa to protect what life there might exist, what difference would it have been where Galileo (RTG and all) collided?

Further, RTGs are designed to survive reentry, and even collision with a hard surface.

The plutonium dioxide (Pu 238) is contained in marshmallow-sized ceramic pellets which are then encased in layers of iridium, then a graphite impact shell and finally an aeroshell. The aeroshell is constructed of material much like a solid cloth with a third dimension of threads going in and out. If one "thread" breaks, the cloth doesn't break; stresses are redistributed and the aeroshell remains intact.

Cassini carries more Pu 238 than Galileo (72 to 48 lbs.) but if its RTG were to disintegrate into the Earth's atmosphere (as was the greatly exaggerated fear of so-called environmentalists at the time of its launch) the expected radiation dosage a person might receive is only one millirem (unit of radioactivity). To put this in perspective, we receive an average of 360 millirems each year just by living on Earth. Luminous dial wristwatches give about 2 millirems (mrem) per year; television from 1 to 10 mrems per year; dental x-rays, 1000 mrem per series; a chest x-ray, 500 to 5000 mrem per exposure.

So, while I am sure there was a discussion regarding the RTGs, I would still think the primary concern was biological contamination of a potentially life-thriving moon.

Rodina

Radiation from the RTG would be virtually unmeasurable in the Jovian system. The unshielded exposure to radiation on, say, Europa is 540 REM/day (not millirem). At Io it's 3,600 REM/day.

Rizz

quote:Originally posted by Robert Pearlman:So, while I am sure there was a discussion regarding the RTGs, I would still think the primary concern was biological contamination of a potentially life-thriving moon.

Considering the fact that we didn't wake up to a pair of sunrises this morning, you are probably correct.

Iím no physicist, but knowing that the Jovian atmosphere is largely made up of Hydrogen (81 percent +/-) and Helium (18 percent +/-) in gas and liquid forms, and considering that Galileo was traveling at approx 100,000 mph +/- as it entered the Jovian atmosphere, after the break up of Galileo, the RTG would have made it a little further along the way towards Jupiter before the intense pressure would have caused the RTG to implode causing possibly some sort of nuclear fission chain reaction.

Not everything is known about Jupiterís composition, pressures, and temperatures. Iím sure that it is safe to assume that an implosion is what occurred to the fuel cylinders at some point in the impact. If the final implosion collapse of the cylinders happened suddenly enough, it might have simulated an explosive-initiated implosion, the similar to a plutonium nuclear bomb.

Personally, I thought that would take precedent over some bio germs.

Rizz

Robert Pearlman

Rizz,

I would suggest reading: Will Galileo Make Jupiter a Star? by badastronomy,com:

Conclusion: Even if you supplied a fission bomb, you won't get hydrogen (tritium) to fuse. And Galileo doesn't have what it takes to make a fission explosion.

Conclusion: Jupiter won't explode, or turn into a star, because it lacks the containment to keep fusion going.

Also, you are making a giant leap by limiting the life on Europa -- if there is life -- to microorganisms. We know from deep sea geyers -- black smokers -- found beneath the waters of the Azores and Nine North, that life -- complex life -- can exist in what should be life-forbidding regions. Its these findings which have scientists so excited about Europa. Given the gravitational forces exerted on the moon by Jupiter, the same tectonic movement needed to create deep sea heat vents on Earth is possible on Europa. Therefore, we may be looking at shrimp, squid, mollusks, and other multi-cell organisms possibly thriving under the water ice of Europa.

[This message has been edited by Robert Pearlman (edited September 22, 2003).]